Heirloom Vegetable: Definition
What do we mean by the term "heirloom vegetable"?
In its broadest sense it refers to a vegetable cultivar that has been grown at some point in human history but which is not grown today in large-scale agriculture.
But there's lots of debate about the specifics of the term.
Heirloom varieties must all be open-pollinated, that is they must grow true to type from collected seed, and they must not be genetically modified.
The other type of vegetable seeds are called modern hybrids (you sometimes see these listed as F1, F2 or F numbered hybrids) and these are varieties that have been created by breeding two or more different varieties together in a specific combination. The seeds produced by the offspring of these crosses are not genetically stable and growing them will result in a wide variety of plants that do not retain the characteristics of the parent.
This is good news for the seed company that developed them, as often only they know the exact combination required to create the hybrid cultivar. This means they can re-sell the seed every year to farmers as the farmers are unable to save the seed. Unforgettably due to modern consumer preferences for uniform looking vegetables, and high productivity requirements, hybrid varieties are often preferred in large-scale agriculture.
Some people like to put a date cutoff for which varieties can be classified as heirlooms. But there's debate here still, some people say a variety should be at least 100 years old, others say that any variety which originates after 1951 can't be classified a heirloom as this was the first year that the widespread sale of hybrid vegetable varieties occurred.
Others say that a true heirloom cultivar should have been bred by one particular family, handed down over the generations, and naturally adapted to the local growing condition of the area where that family resided.
I prefer to use a simpler definition, I regard any variety of vegetable that will grow true to type from collected seeds to be a heirloom cultivar, regardless of when that variety was first bred and stabilized. This definition is useful as it allows for recently developed, stabilized, true to type cultivars to also be regarded as a new heirloom variety for the future.
We should not disregard any recently developed heirloom varieties as essentially, the only difference between any heirloom variety and a modern hybrid is that the genetics of the heirloom have been stabilized to grow true to type from collected seed each year, while planting the collected seed of the hybrid will result in offspring with a wide variance of characteristics, most of which less favorable than the original plant.
There is another term "heritage vegetable" that can be used for older heirloom cultivars of significance. All heritage varieties are heirloom varieties, but not all heirlooms varieties are heritage varieties.